Radar has countless applications in modern life. Due to the continuing reduction in size and cost of digital signal processing chips, radar is now being incorporated into consumer products. For instance, radar technology has become sufficiently low cost to be incorporated into cars and other motor vehicles to provide parking assistance systems, collision avoidance systems, and air bag deployment systems (pre-crash detection).
It might be desirable to optimize the parameters of the radar system differently depending on the particular task being performed by the radar. Using vehicle-mounted radar systems as an example, in parking assistance, the radar is used to warn the driver how close the vehicle is to obstacles (including small obstacles that the driver may not be able to see through the rear window or the mirrors). Accordingly, in parking assistance, only obstacles that are relatively close to the vehicle, e.g., within 2 m, are of interest. Hence, the distance over which the radar must perform is very small, typically between 0-2 m. On the other hand, it is important to detect relatively small obstacles such as parking meter poles, fire hydrants, and curbs. Thus, the range spectrum of the radar can be small. However, on the other hand, the range resolution of the radar (in terms of the ability to detect small objects, resolve multiple, closely-spaced objects from each other and determine the range to an obstacle within a few cm) must be high. For instance, when parking, the difference between an object being 20 cm or 50 cm from the rear bumper of the vehicle is a significant difference. Also, Doppler resolution, i.e., resolution of the velocity of an obstacle relative to the car may not be that significant for parking assistance since the vehicle typically is moving relatively slowly during parking.
On the other hand, when a vehicle-mounted radar system is used for collision avoidance, an entirely different set of concerns are significant. For instance, in a collision avoidance system, the radar is used to detect moving vehicles and other obstacles at much greater distances, such 30, 60, or even 100 m from the car, and the relative velocity of obstacles can be much greater, e.g., 100 km/h or greater, as compared to parking assistance. On the other hand, the high range resolution of a few centimeters or so that is desirable for parking assistance is not required for collision avoidance applications.
Accordingly, optimizing the parameters of the radar system for one of these two exemplary applications of a vehicle-mounted radar system almost inherently dictates that it is not optimized for the other application.
The following discussion focuses on the issue of determining the range to an object and the relative velocity of the object. However, it should be understood that virtually any practical radar system also will employ additional processing and/or hardware to determine the bearing to an object, which is not significantly discussed in this specification.
There are many different well-known techniques for processing radar signals to determine the range of objects, to resolve different objects from each other, and to determine the relative velocity of objects. For instance, in pulsed radar systems, the radar sends out a transmit pulse and waits for a reflection to come back. The reflected signal is fed into one input port of a mixer and the transmitted signal is fed into the other input port of the mixer after being passed through a delay line. The output of the mixer is converted to digital by an analog-to-digital converter and the digital information is processed in a digital signal processor to determine the delay between the transmission of the pulse and the receipt of the reflection, which, of course, is directly convertible into the round trip distance between the radar antenna and the obstacle that reflected the signal by multiplying the delay by the speed of light. In pulsed radar, the amount of digital signal processing is relatively small, since the output of the hardware (the output of the mixer) requires relatively little further processing to derive range information.
Initially, pulsed radar systems were favored for consumer and other low-cost radar applications because pulsed radar systems were hardware intensive, as opposed to processing intensive. In other words, a pulsed radar system requires relatively little processing power because most of the work is performed by hardware, e.g., switches, oscillators, delay lines, and mixers. However, as digital signal processing technology continues to improve at a rapid pace, radar techniques that rely more heavily on digital signal processing, rather than hardware, are becoming more and more attractive. The cost of hardware remains relatively steady, while the cost of digital signal processing decreases at a rapid pace.
One radar modulation technique that requires relatively little hardware, but significant signal processing power, and that is becoming more popular for consumer-type radar applications as digital signal processing becomes less and less expensive, is the frequency modulated continuous wave (FMCW) technique. There are several different types of FMCW radar modulation techniques. However, they all have in common the fact that the radar transmit signal is frequency modulated over time. Some of the better-known FMCW radar modulation techniques are frequency shift keying (FSK) and stepped frequency modulation.
In classical FMCW modulation techniques, the transmit signal of the radar is continuously swept from a minimum frequency to a maximum frequency over a period of time. This is commonly called a chirp. The chirp is repeated a plurality of times and the reflection information from the plurality of chirps is collected, correlated, and processed to generate sufficient data to permit calculation of useful results, such as the number of objects in the field of view of the radar, their ranges, bearings, sizes, and/or velocities.
Another FMCW modulation technique is FSK (frequency shift keying). In radar systems that utilize FSK modulation techniques, the radar sequentially transmit signals of two different frequencies. The signals reflected off of an obstacle back to the radar will have a certain phase difference relative to the corresponding transmit signals, which phase difference depends on the distance to the object (let us assume for sake of simplicity that the object is stationary, since velocity also would affect the phase difference). This is true for each of the two transmit frequencies transmitted by the radar. The phase difference between the transmitted signal and the received signal for the first frequency signal and the phase difference between the transmitted signal and the received signal for the first frequency signal are different from each other.
The phase difference between the transmitted signal and the reflected signal for any one frequency does not provide enough information to determine the range to an obstacle. Particularly, the difference in phase between the transmitted signal and the received signal does not disclose how many wavelength cycles exist in the round-trip delay between transmitted signal and received reflection signal. In other words, the phase difference data provides fine tuning with respect to range resolution (e.g., a phase difference of 180° discloses that the obstacle is X.5 wavelengths away—roundtrip—, but does not disclose the value of the integer X). However, phase difference information at two or more different frequencies can be correlated to each other to determine range.
With only two transmit frequencies, FSK modulation techniques cannot provide range measurement data unless there are at least two objects within the field of view of the radar that are moving at different velocities relative to the radar. However, by increasing the number of frequencies transmitted, one can increase the number of obstacles that can be resolved. Radar modulation techniques that utilize many frequency steps per measurement cycle are known as stepped frequency modulation techniques.
The above discussion assumed that the obstacles are not moving. In a real-world situation in which the obstacles may be moving or stationary, another layer of complexity in the signal processing is introduced. Particularly, by means of the well-known Doppler Effect, when an obstacle is moving relative to the radar antenna, the signal reflected off that object will be shifted in frequency from the transmitted signal. This frequency shift also will alter the perceived phase difference between the transmitted signal and the corresponding received, reflected signal.
Accordingly, even more complex modulation techniques and signal processing is necessary to provide sufficient information to distinguish the portion of the phase difference data that is the result of the Doppler Effect from the portion of the phase difference data that is the result purely of the range to the detected obstacle.
One well-known technique for distinguishing the Doppler effect from the range effect is to employ a sort of frequency chirping technique in which the set of frequency steps (hereinafter a frequency step cycle) is repeated a number of times sequentially, with each frequency step cycle being shifted in frequency from the preceding frequency step cycle. A first plurality of these sequential frequency step cycles will be shifted in frequency relative to each other in a first sequential direction, e.g., up. This will be followed by a second plurality of frequency step cycles shifted in frequency relative to each other in the opposite direction, e.g., down. The set of up-chirped frequency step cycles plus the set of down-chirped frequency step cycles collectively comprises one complete measurement data set (from which the range and velocity of obstacles can be calculated).
Merely as an example, the first frequency step cycle may comprise sweeping the frequency of the transmitted signal over a range of 200 MHz, from 4.000 GHz to 4.200 GHz, linearly in 10 steps of 20 MHz each. Thus, the first stepped frequency cycle comprises transmit signals at 4.0000 GHz, 4.020 GHz, 4.040 GHz, 4.060 GHz, . . . 4.160 GHz, 4.180 GHz, and 4.200 GHz. The second stepped frequency cycle comprises the same number of frequency steps, i.e., 10, each frequency step separated from the preceding frequency step by the same 20 MHz and the cycle spanning the same range of 200 MHz, but instead starting at 4.100 GHz. This continues for a number of stepped frequency cycles, e.g., five. For instance, the third cycle would start at 4.200 GHz, the fourth cycle would start at 4.300 GHz, and the last cycle would start at 4.400 GHz.
Next, this will be followed by a plurality of down-chirped stepped frequency cycles. For instance, the next frequency step cycle starts again at 4.0000 GHz and comprises transmit signals at 4.0000 GHz, 4.020 GHz, 4.040 GHz, 4.060 GHz, 4.080 GHz, . . . , 4.160 GHz, 4.180 GHz, and 4.200 GHz. The following stepped frequency cycle comprises the same number of frequency steps, each frequency step separated from the preceding frequency step by the same 20 MHz and spanning the same range of 200 MHz, but instead starting at 3.9000 GHz. This continues for a number of stepped frequency cycles. For instance, the third cycle would start at 3.800 GHz, the fourth cycle would start at 3.700 GHz, and the last cycle would start at 3.600 GHz.
If an object is moving relative to the radar, the observed phase rotation for that obstacle will be different than if it was stationary for each different stepped frequency cycle in one and only one of the up-chirped set of stepped frequency cycles or the down-chirped set of stepped frequency cycles. Whether that change in phase rotation appears in the up-chirped sequence or down-chirped sequence depends on whether the obstacle is moving toward or away from the radar antenna. In any event, that difference will be the result of only the Doppler Effect because the portion of the overall phase rotation that is a function of the range to the object is unaffected by the up or down chirping of the stepped frequency cycles. Accordingly, the portion of the phase rotation that is the result of the Doppler Effect can be isolated by comparing the data from the up chirped sequence of stepped frequency cycles to the data from the down-chirped sequence of stepped frequency cycles.
Hence, it is possible to determine the, range and velocity of a plurality of obstacles in the field of view of the radar.
While frequency up and down chirping provides the information necessary to differentiate velocity from range, it decreases the signal to noise ratio of the overall result for a given ramp rate of the chirped signal and velocity of the object by about 3-6 dB because the amount of time over which the data can be integrated is cut in half. Particularly, in one complete measurement cycle, half of the time is spent up chirping and half of the time is spent down chirping and the data from the up chirping sequence must be processed largely separately from the data from the down chirping sequence.
Some of the many advantages of FMCW techniques over pulsed radar techniques include the fact that, although the transmit signal generally has a broader bandwidth than the transmit signal in pulsed radar modulation techniques, the bandwidth of the post-mixer signals is typically much narrower, which reduces the data acquisition speed required of the signal processing circuitry. Accordingly, FMCW is less prone to interference from other RF sources and is much less prone to causing interference in other RF receivers.
While the preceding discussion has been at the conceptual level, it will be understood by those of skill in, the art of radar signal processing that the required signal processing is actually very processor intensive. For instance, much of the information used in determining the range and velocity information comprises phase data taken over a plurality of time and frequency intervals. It is difficult to process such phase information in the time domain. Accordingly, it is common to convert the phase information collected over the plurality of samples into a different form, such as a histogram or into the frequency domain (e.g., via Fourier transform) and process the data in the frequency domain before ultimately converting back to the time domain and/or generating range and velocity information.
It should be clear from the discussion above that the most relevant information collected in the stepped frequency modulation technique essentially is the changes in phase of the reflected signals over time and transmit signal frequency (often referred to in the related industries as “phase rotation”). It will be appreciated by those of skill in the related arts that a change in phase over time (i.e., phase rotation) essentially is a “frequency”. Therefore, converting the data to the frequency domain, such as by Fast Fourier Transform (FFT), directly yields range and velocity information of the obstacles in the field of view of the radar.
The frequency corresponding to this change in phase shall hereinafter be termed the phase rotation frequency. As noted above, there are two phenomena that collectively dictate the phase rotation frequency, namely, the range of the object and the relative velocity of the object (or Doppler shift). For purposes of clarity and ease of discussion, the portion of that frequency that is the result of Doppler shift (i.e., the velocity of an obstacle) is referred to herein as the Doppler frequency and the portion that is the result of the range of the obstacle is herein referred to as the range frequency.